In the aforementioned copending application there is shown a convection oven in which combustion products from a gas burner may be circulated through an oven by mixing said combustion products with vapor drawn out of the oven into the inlets of a circulating fan system. As the capacity of the burner is increased, the flame length of such a burner increases to a point where combustion is not completed before the flame enters the region between the inlet of the blower and the screened outlet of the oven. The vapor can thus become ionized and can act as a partial conductor at the screened region outlet which can permit microwave energy to be emitted from the interior of the oven into the region of the burner blower. This can result in absorption of the microwave energy and hence reduction of microwave energy cooking power in the oven, and can allow the microwave energy to leak through the burner and/or blower structure out into the room.